It has been found that the wave field synthesis method for reproducing audio signals as first described in 1988 by Prof. Berkhout [1] can be used to physically reconstruct wave fronts originating from a natural sound source according to the Huygens principle. A virtual sound source is created at the location of the natural sound source from the elementary waves of a large number of individually activated sound transducers.
If these sound transducers are arranged on a two-dimensional surface, the principle of wave field synthesis creates the “acoustic curtain” phenomenon. In this area, all sound sources, and even the reflections of these sound sources can be physically reconstructed in all three spatial dimensions in the recording space, and the acoustics of the recording space is created.
In order to fully reconstruct the acoustic conditions in the recording space, it would be necessary to set up the acoustic curtain to surround the listener so that all reflections of the recording space can also be generated at their proper points of origin. In practice, however, it has not yet been proven possible to create such a “sound booth”. The number of sound transducers would become very large, because they must be arranged close together to prevent the aliasing effects that would occur otherwise.
In practice, therefore, the wave field synthesis method is usually reduced to a horizontal row of sound transducers, which are arranged so as to surround the listener. This also has the effect of reducing the reproduction to this horizontal plane. Therefore, correct spatial playback is no longer possible. Moreover, the acoustics of the reproduction space must then be suppressed entirely due to the cylindrical propagation form of the wave fronts.
In the last few years several research facilities have successfully created a two-dimensional acoustic curtain. A solution, in which not all reflections but only the psychoacoustically significant direct wave fronts and the first, acoustically intensive reflections are reconstructed at their correct point of origin in the recording space, has been described in [3], according to which, in a model-based approach the sound transducers arranged to surround the listener are replaced with selectively generated reflections of the reproduction space.
However, it is also hardly feasible to position such an acoustic curtain directly in front of a listener if it is constructed as a single unit. It must be big enough to reproduce the direct wave fronts in its range. The cost of this would be enormous. Apart from this, it would be very difficult to transport it into the reproduction space as a fully assembly unit.
If several systems are coupled together, the computational complexity for wave field synthesis of a limited number of virtual sound sources with fixed positions is still manageable, even when constructing a two-dimensional acoustic curtain. But if a sound source moves in the recording space, all of the travel times and all levels of all of the reflections depending thereon must be recalculated for each individual sound transducer. The task of executing this operation for all sound transducers in an acoustic curtain fast enough to be able to continually represent such movement of a sound source approaches the current technological limits—even in the model-based approach of wave field synthesis and if only reflections of the first order are considered.
The computational complexity increases significantly if an attempt is made to represent a listener's change of location in the recording space. Then, all travel times of all direct wave fronts and of all reflections at every single sound transducer are also changed. The recalculated data would have to be read in at least eight times per second in order to be able to reflect a somewhat fluid movement [4].
Therefore, given the amount of computing power that is available, in the database-driven approach of wave field synthesis, researchers are resorting to calculating the pulse responses for each sound transducer for discrete source positions in advance and storing them, so that the virtual sound sources can then be rapidly shifted from one position to another [5].
However, rapid movements of the virtual sound source do not generate the Doppler effects that occur when a real sound source changes location.